The present invention relates to a serial printer, and more particularly to a method and apparatus for controlling the movement of a printing head of a printer of the type which prints characters while reciprocally moving the printing head.
In the serial printer which prints characters while moving the printing head along character lines, a print speed is increased by using both the forward and backward movements of the printing head for printing. When this bidirectional printing method is used to print an enlarged character higher than the height of the printing head through a plural number of scans by the printing head and at least one vertical feed of a recording paper, which corresponds in length to the print width, a resulting printed image can be discontinuous at the boundary of the scan zone due to mechanical error of the printer. Accordingly, its image quality is poor.
In printing such a character divided into zones for printing, deterioration of the print quality can be prevented by moving the printing head in one direction (e.g., left to right) during printing.
This type of a printer is disclosed in Unexamined Japanese Patent Publication No. Sho. 61-57348.
FIG. 11 is an illustration that facilitates understanding of the print control, viz., the movement of a printing head and the paper feed control in such a conventional printer. As shown, a series of enlarged characters "A B C" are printed on the m-th line, and another series of characters of standard size "h i j k" are printed on the n-th line spaced by a distance l from the m-th line.
In a first step of the print control, a carriage carrying the printing head is first moved in one direction (e.g., from left to right). Through the movement of the carriage, the upper half of the enlarged character series, "A B C", is printed on a line. Then, a recording paper is fed by the distance l. In a second step, the standard character series, "h i j k", is printed. Thereafter the recording paper is fed by a distance of (H-1).
Subsequently, the lower half of the enlarged character series, "A B C", is printed in the same direction (left to right) as in the printing operation of the upper half.
Thus, according to the print control in the conventional printer, those characters to be divided into a plurality of zones for their printing are printed unidirectionally. Accordingly, the print control succeeds in eliminating the problem of the discontinuity in the printed characters, which arises from the different printing directions of the printing head.
In the print control disclosed in the Unexamined Japanese Patent Publication No. Sho. 61-57348, the character images that are printed through one movement of the printing head are those characters occupying only one line. For this reason, when this print control is used to print character images of a plurality of lines in a state where these overlap in a scan zone, as viewed in the moving direction of the printing head, the number of movements of the printing head and the number of feeds of the recording paper are increased. As a result, the printing speed is decreased.